1. Field of the Invention
The present invention relates to a fundus camera for photographing a fundus of an eye to be examined, and more particularly to a focusing mechanism of the fundus camera suitable for focusing a fundus image illuminated by an infrared light.
2. Description of Related Art
The conventional fundus camera for photographing the fundus of the eye has been well-known as a non-mydriasis method of photographing the fundus image by using an infrared light as an illumination light for observation without using an additional medicine like a mydriatica. However, since the fundus image illuminated by the infrared light mainly seemed to be an image of blood vessel, the whole image was less contrasty and indistinct. As a result of this, the focusing was very difficult by only judging such a fundus image.
In order to solve this problem, by providing a target projecting optical system for projecting a target for focusing onto the fundus area, the target projecting optical system is coaxial with an optical axis of an illumination optical system for observation by an oblique mirror. Consequently, the apparatus for judging the suitability of focusing of the fundus image by observing a state of the target image projected on the fundus has been proposed. Since the target image for focusing is not necessary at photographing, it is structured that the target is not be projected by withdrawing the oblique mirror arranged on the optical axis of the illumination optical system.
However, it was necessary for the above-mentioned structured apparatus to provide a light source for projecting a target, a target plate, a relay lens, an oblique mirror and the like apart from the illumination optical system for observation. Also, since another mechanism of withdrawing the oblique mirror at photographing was needed, it resulted in a disadvantage that the whole structure was complicate. In view of this problem, it may be demerit for a handheld type of fundus camera to achieve compactness and lightening.
Further, if a focused range is enlarged, it may result another disadvantage that the difficulty of determining a focusing direction may cause unnecessary time for operating the apparatus.